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Active sensing
Ehud Ahissar 1
Active sensing
• Passive vs active sensing (touch)
• Comparison across senses
• Basic coding principles
--------
• Perceptual loops
• Sensation-targeted motor control
• Proprioception
• Controlled variables
• Active vibrissal touch: encoding and recoding
2
Eye movements during fixation
3
=> Temporal coding (“when are receptors activated”)
Sensory organs consist of receptor arrays:
audition
10 mm
cochlea
vision
retina
10 mm
somatosensation
Finger pad
~200 mm
Spatial organization => Spatial coding (“which receptors are activated”)
Movements
sensory encoding: What receptors tell the brain
4
Temporal coding in action
5
Coding space by time
1. Spatial frequency
2. Spatial phase
6
Darian-Smith & Oke,
J Physiol, 1980
anesth. monkey,
MR fibers
Touch: Temporal encoding of spatial features
7
RA fiber
Vel - constant
f = SF * V
dt = dx / V
8
SA fiber Vel
SF
9
RA fiber
V1 V2 V3
G1
G2
G3
Vel SF
10
PC fiber Vel SF
11
Coding ranges
12
Coding space by time
1. Spatial frequency
2. Spatial phase
13
space
time
retinal outputs
1
2
spac
e
Vision: Temporal encoding due to eye movement
V eye
RF(1)
RF(2)
14
space Dx
Dt time
retinal outputs
1
2
spac
e
V eye
RF(1)
RF(2)
Vision: Temporal encoding due to eye movement
15
space Dx
Dt time
retinal outputs
1
2
spac
e
V eye
RF(1)
RF(2)
Vision: Temporal encoding due to eye movement
16
space Dx
Dt time
retinal outputs
1
2
spac
e
V eye
RF(1)
RF(2)
Vision: Temporal encoding due to eye movement
17
space Dx
Dt time
retinal outputs
1
2
spac
e
V eye
RF(1)
RF(2)
Vision: Temporal encoding due to eye movement
18
Spatial vs temporal coding
• scanning allows sensing in between receptors
Spatial Temporal
faster
better resolution
19
Passive vs Active sensing
Passive Active
threshold low high
accuracy low high
Systems involved sensory Sensory + motor
coding spatial Spatial + temporal
Processing speed fast slow
of stationary objects
Used in detection Exploration
Localization
Identification
… 20
where touch begins?
Central processing of touch
Text book: at the receptors
21
Motor Sensory
Brainstem Loop
Facial Nucleus
-
Zona Incerta
ex
trale
mn
isca
l
Cerebellar/Olivary
V L Thalamic Nuclei
Vibrissae
Thalamus
Cortex
POm
Trigeminal Ganglion
Primary
Motor Cortex Secondary
Superior Colliculus
+
Red Nucleus
Reticular Formation
Brainstem Reticular
Nucleus
Pontine
Primary Sensory Cortex
VPM-dm
VPM-vl
Trigeminal
Nuclei
Sensory-motor loops of the vibrissal system
22
WT
W T
WT T W
a m A
B
C
D
E
Localization (‘where’)
Identification (‘what’)
Whisking
Brainstem
Thalamus
Sensory-motor loops of the vibrissal system
Cortex
The old view
23
Motor Sensory
Brainstem Loop
Facial Nucleus
-
Zona Incerta
ex
trale
mn
isca
l
Cerebellar/Olivary
V L Thalamic Nuclei
Vibrissae
Thalamus
Cortex
POm
Trigeminal Ganglion
Primary
Motor Cortex Secondary
Superior Colliculus
+
Red Nucleus
Reticular Formation
Brainstem Reticular
Nucleus
Pontine
Primary Sensory Cortex
VPM-dm
VPM-vl
Trigeminal
Nuclei
Sensory-motor loops of the vibrissal system
24
WT
W T
WT T W
a m A
B
C
D
E
Localization (‘where’)
Identification (‘what’)
Whisking
Brainstem
Thalamus
Cortex
25
WT
W T
WT T W
a m A B
C D
E
Sensory-motor loops of the vibrissal system
26
where touch begins?
Central processing of touch
Text book: at the receptors
27
Active touch does not begin at the receptors
Sensor motion determines the interaction between the receptors and external objects
Motor control
• Closed loops
• Proprioceptive feedback
• Reflexes – tool for probing loop function
• Controlled variables – motor vs sensory
28
Motor control
• Closed loops
• Proprioceptive feedback
• Reflexes – tool for probing loop function
• Controlled variables – motor vs sensory
29
30
Excitation Contraction Coupling
Phase 1:
Firing of Motor Neuron
Phase 2:
Release of Neurotransmitter
31
Excitation Contraction Coupling
Phase 1:
Firing of Motor Neuron
Phase 2:
Release of Neurotransmitter
Phase 3:
Muscle contraction
32
Open-loop system
Information flows in one direction (from neurons to muscles
33
Closed-loop system
Information flows in a closed loop: from neurons to muscles and from muscles to neurons
What kind of information ?
Open-loop system
Information flows in one direction (from neurons to muscles
34
Closed-loop system
The direct feedback from muscles and joints is mediated by proprioceptive signals
muscle length Sensitive to: muscle tension Flexion, extension
Proprioceptive receptor types
Muscle spindle
receptors Name: Golgi tendon
organs
Joint receptors
35
Proprioceptive receptor types
Muscle spindle
receptors
muscle length
Name: Golgi tendon
organs
muscle tension
Joint receptors
Flexion, extension
Location: Fleshy part of the
muscle
Between muscle and
tendon
Joint capsule
Parallel to muscle
fibers
Serial to
muscle fibers
Between bones
Sensitive to:
Motor control
• Closed loops
• Proprioceptive feedback
• Reflexes – tool for probing loop function
• Controlled variables – motor vs sensory
36
37
What proprioceptors encode?
38
Proprioceptive receptor types
Muscle spindle
receptors
muscle length
Name: Golgi tendon
organs
muscle tension
Joint receptors
Flexion, extension
From
Arthur Prochazka,
University of Alberta
Sensitive to:
39
Proprioceptive receptor types
Muscle spindle
receptors
muscle length Sensitive to:
Name: Golgi tendon
organs
muscle tension
Joint receptors
Flexion, extension
Encode: force
f = k1F
40
Proprioceptive receptor types
Muscle spindle
receptors
muscle length Sensitive to:
Name: Golgi tendon
organs
muscle tension
Joint receptors
Flexion, extension
Length + velocity
f = k1L + k2 V 0.6 Encode:
force
f = k1F
angle
f = k1q
41
Proprioceptive receptor types
Muscle spindle
receptors
muscle length Sensitive to:
Name: Golgi tendon
organs
muscle tension
Joint receptors
Flexion, extension
Length + velocity
f = k1L + k2 V 0.6 Encode:
force
f = k1F
angle
f = k1q
q q q
42
PID control
• Proportional (to the controlled variable)
• Integral (of the controlled variable)
• Derivative (of the controlled variable)
Present
Past
Future
q
q
q
43
Negative feedback loop
• Characteristic: The effect of a perturbation is in opposite direction
• Requirement: The cumulative sign along the loop is negative
• Function: Can keep stable fixed points
44
Positive feedback loop
• Characteristic: The effect of a perturbation is in the same direction
• Requirement: The cumulative sign along the loop is positive
• Function: amplifies perturbations
+
Motor control
• Closed loops
• Proprioceptive feedback
• Reflexes – tool for probing loop function
• Controlled variables – motor vs sensory
45
46
The stretch reflex probes the control function of
muscle spindles
47
Is the loop positive or negative?
The stroke stretches the spindle
As a result the muscle contracts
The result opposes the perturbation
=> negative FB loop
48
the anatomical loop
Muscle spindle excites the motor neuron
Motor neuron excites muscle fibers
Muscle contraction suppresses spindle response
49
Proprioceptive receptor types
Muscle spindle
receptors
muscle length Sensitive to:
Name: Golgi tendon
organs
muscle tension
Joint receptors
Flexion, extension
Encode: force
f = k1F
Why spindles fire at rest?
50
What about the flexor muscles?
Positive or negative loop?
What is the underlying circuit?
51
Pain reflex
Positive or negative?
What is the underlying circuit?
Motor control
• Closed loops
• Proprioceptive feedback
• Reflexes – tool for probing loop function
• Controlled variables – motor vs sensory
52
Motor Sensory
Brainstem Loop
Facial Nucleus
-
Zona Incerta
ex
trale
mn
isca
l
Cerebellar/Olivary
V L Thalamic Nuclei
Vibrissae
Thalamus
Cortex
POm
Trigeminal Ganglion
Primary
Motor Cortex Secondary
Superior Colliculus
+
Red Nucleus
Reticular Formation
Brainstem Reticular
Nucleus
Pontine
Primary Sensory Cortex
VPM-dm
VPM-vl
Trigeminal
Nuclei
Sensory-motor loops of the vibrissal system
53
Basic principles of closed-loop control
54
f
V
Vs0
Vm0
Vm=f(-Vs) Vs=g(Vm)
Set point
Vm Vs
Vm
Vs
Vd +
-
55
f
V
Vs0
Vm0
Vm=f(Vd-Vs) Vs=g(Vm)
Vm
Vs
Vd +
-
Vsd
Vmd
Vm Vs
Set point
56
f
V
Vs0
Vm0
Vm=f(Vd-Vs) Vs=g(Vm)
Direct control without direct connection
Vm
Vs
Vd +
-
Vsd
Vmd
Vm Vs
57
f
V
Vs0
Vm0
Vm=f(Vd-Vs) Vs=g(Vm)
Nested loops
Vm
Vs
Vd +
-
Vsd
Vmd
f2
+
-
Vm Vs
58
f
V
Parallel loops
-
Vs0
Vm0
Vm1=f(Vd-Vs) Vs=g(Vm1)
Vm1
Vs
Vsd
Vmd
f2
+
-
Vm1 Vs
Vm2
Vs0
Vm0
Vm2=f(Vd-Vs) Vs=g(Vm2)
Vm2
Vs
Vsd
Vmd
+
59
f
V
Parallel loops
-
Vs0
Vm0
Vm=f(Vd-Vs) Vs=g(Vm)
Vm1
Vs
Vsd
Vmd
f2
+
-
Vm Vs
Xm
Xs0
Xm0
Xm=f(Xd-Xs) Xs=g(Xm)
Xm
Xs
Xsd
Xmd
+
Xs
60
Closed loops in active sensing
f
V
-
f2
+
-
Vm Vs
Xm
+
Xs
• Motor (via Xs)
(velocity, amplitude, duration, direction, …)
• Sensory (Xs)
(Intensity, phase, …)
• Object (via Xm – Xs relationships)
(location, SF, identity, …)
The controlled variables can be
61
Motor Sensory
Brainstem Loop
Facial Nucleus
-
Zona Incerta
ex
trale
mn
isca
l
Cerebellar/Olivary
V L Thalamic Nuclei
Vibrissae
Thalamus
Cortex
POm
Trigeminal Ganglion
Primary
Motor Cortex Secondary
Superior Colliculus
+
Red Nucleus
Reticular Formation
Brainstem Reticular
Nucleus
Pontine
Primary Sensory Cortex
VPM-dm
VPM-vl
Trigeminal
Nuclei
Sensory-motor loops of the vibrissal system
63
Motor Sensory
Brainstem Loop
Facial Nucleus
-
Zona Incerta
ex
trale
mn
isca
l
Cerebellar/Olivary
V L Thalamic Nuclei
Vibrissae
Thalamus
Cortex
POm
Trigeminal Ganglion
Primary
Motor Cortex Secondary
Superior Colliculus
+
Red Nucleus
Reticular Formation
Brainstem Reticular
Nucleus
Pontine
Primary Sensory Cortex
VPM-dm
VPM-vl
Trigeminal
Nuclei
Sensory-motor loops of the vibrissal system
64
Motor Sensory
Brainstem Loop
Facial Nucleus
-
Zona Incerta
ex
trale
mn
isca
l
Cerebellar/Olivary
V L Thalamic Nuclei
Vibrissae
Thalamus
Cortex
POm
Trigeminal Ganglion
Primary
Motor Cortex Secondary
+
Red Nucleus
Reticular Formation
Brainstem
Primary Sensory Cortex
VPM-dm
VPM-vl
Trigeminal
Nuclei
Sensory-motor loops of the vibrissal system
Superior Colliculus
Reticular Nucleus
Pontine
65
Motor Sensory
Brainstem Loop
Facial Nucleus
-
Zona Incerta
ex
trale
mn
isca
l
Cerebellar/Olivary
V L Thalamic Nuclei
Vibrissae
Thalamus
Cortex
POm
Trigeminal Ganglion
Primary
Motor Cortex Secondary
+
Red Nucleus
Reticular Formation
Brainstem
Primary Sensory Cortex
VPM-dm
VPM-vl
Trigeminal
Nuclei
Sensory-motor loops of the vibrissal system
Superior Colliculus
Reticular Nucleus
Pontine
66
Active sensing in
the vibrissal system
67
Sensory signal conduction
The vibrissal system
68
Sensory signal conduction
The vibrissal system
whisker
Meisner
Merkel
Ruffini
Lanceolate
free endings
69
70
Motor Sensory
Brainstem Loop
Facial Nucleus
-
Zona Incerta
ex
trale
mn
isca
l
Cerebellar/Olivary
V L Thalamic Nuclei
Vibrissae
Thalamus
Cortex
POm
Trigeminal Ganglion
Primary
Motor Cortex Secondary
Superior Colliculus
+
Red Nucleus
Reticular Formation
Brainstem Reticular
Nucleus
Pontine
Primary Sensory Cortex
VPM-dm
VPM-vl
Trigeminal
Nuclei
Sensory-motor loops of the vibrissal system
71
Motor control of whiskers
Intrinsic muscles
Dorfl J, 1982, J Anat 135:147-154
72
Follicle as a motor-sensory junction
• Motor signals move the follicle and whisker
• Follicle receptors report back details of self motion
• Plus perturbations of this motion caused by the external world
Dorfl J, 1985, J Anat 142:173-184
= proprioception
73
Reception of neuronal signals in the brain
Afferent signals that relate to the external world contain:
• Reafferent (self-generated) sensory signals
• Exafferent (externally generated) sensory signals
Exteroception – reception of the external world via the
six senses: sight, taste, smell, touch, hearing, and balance
Interoception: reception of the internal organs of the
body
Proprioception (from "one's own" and reception)
reception of the relationships between the body and the
world.
74
Proprioceptive receptor types
Muscle spindle
receptors
muscle length
Name: Golgi tendon
organs
muscle tension
Joint receptors
Flexion, extension Sensitive to:
75
Motor control of whiskers
Intrinsic muscles
Dorfl J, 1982, J Anat 135:147-154
76
Vibrissal proprioception
• Each follicle contains ~2000 receptors
• About 20% of them convey pure
proprioceptive information
77
Proprioceptive loop Proprioceptive loop
Skeletal system Vibrissal system
78
Whiskers come with different muscle sizes
Intrinsic muscles
Dorfl J, 1982, J Anat 135:147-154
0.5 mm
79
Whisking behavior – reflections of control loops
80
Perception of external objects
• What signals must the brain process in order to
infer a location of an external object in space?
• Reafferent + exafferent signals
Object localization
84
What the whiskers tell the rat brain
Touch
Exafference:
Their own movement
Reafference:
(“Whisking”)
86
Whisker position vs. time space
time
Whisking
What the whiskers tell the rat brain
87
Whisker position vs. time space
time
Whisking
What the whiskers tell the rat brain
88
Whisker position vs. time space
time
Whisking
What the whiskers tell the rat brain
89
Whisker position vs. time space
time
Whisking
What the whiskers tell the rat brain
90
Whisker position vs. time space
time
Whisking
What the whiskers tell the rat brain
91
Whisker position vs. time space
time
Whisking
What the whiskers tell the rat brain
92
Whisker position vs. time space
time
Whisking
What the whiskers tell the rat brain
93
Whisker position vs. time space
time
Whisking
What the whiskers tell the rat brain
94
Whisker position vs. time space
time
Whisking
What the whiskers tell the rat brain
95
Whisker position vs. time space
time
Whisking
What the whiskers tell the rat brain
96
Whisker position vs. time space
time
Whisking
What the whiskers tell the rat brain
97
What the whiskers tell the rat brain
Touch
Exafference:
Their own movement
Reafference:
(“Whisking”)
98
Whisker position vs. time space
time
Touch
What the whiskers tell the rat brain
99
Whisker position vs. time space
time
Touch
What the whiskers tell the rat brain
100
Whisker position vs. time space
time
Touch
What the whiskers tell the rat brain
101
Whisker position vs. time space
time
Touch
What the whiskers tell the rat brain
102
Whisker position vs. time space
time
Touch
What the whiskers tell the rat brain
103
Whisker position vs. time space
time
Touch
What the whiskers tell the rat brain
104
Whisker position vs. time space
time
Touch
What the whiskers tell the rat brain
105
Whisker position vs. time space
time
Touch
What the whiskers tell the rat brain
106
Whisker position vs. time space
time
Touch
What the whiskers tell the rat brain
107
Whisker position vs. time space
time
Touch
What the whiskers tell the rat brain
108
Whisker position vs. time space
time
Touch
What the whiskers tell the rat brain
109
• Whisking:
What the whiskers tell the rat brain
• Touch:
Whisker position vs. time space
time
Whisker position vs. time contact with object
space
time
How can the brain use this information?
110
• Whisking:
• Touch:
Whisker position vs. time space
time
contact with object
space
time
Whisker position vs. time
?
?
What the whiskers tell the rat brain
How can the brain use this information?
111
• Whisking:
How can the brain extract the location of the object
• Touch:
contact with object
Whisker position vs. time space
time
112
• Whisking:
How can the brain extract the location of the object
• Touch:
contact with object
Whisker position vs. time space
time
113
=> Temporal coding (“when are receptors activated”)
Sensory organs consist of receptor arrays:
audition
10 mm
cochlea
vision
retina
10 mm
somatosensation
Finger pad
~200 mm
Spatial organization => Spatial coding (“which receptors are activated”)
Movements
sensory encoding: What receptors tell the brain
114
Orthogonal coding of object location
• Horizontal object position is encoded by time
• Radial object position is encoded by rate
• Vertical object position is encoded by space
115
Active sensing
The End 116